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May 12

Enthalpy of Reaction for Ethylene and Water

I'm working through the problems in my Organic Chemistry text book, and came across one where I'm confused as to what has gone wrong. The question and solution are shown here. The question asks me to calculate the enthalpy of reaction for ethylene and water, and then state whether it is exothermic or endothermic. Using the values stated in the table (and also given in their solution shown in the image I linked) I calculated the enthalpy as +39KJ/mol, and thus characterize the reaction as endothermic. In their solution, they appear to have gotten the sign backwards and have said the result is -39KJ/mol and therefore characterized it as exothermic. I tried checking the known value for the enthalpy of the reaction, and various sources give -44KJ/mol and state it is exothermic. So the solution given in the text conforms closely to the known value but the reasoning seems wrong. One thing to note is that there is no value for the bond dissociation enthalpy of the C-O bond created in ethanol in the table they reference, but both in my answer and in the text book's solution, we simply used the BDE value for the C-O bond in methanol. If this bond has a significantly different BDE to the one we actually need, then it might explain what's gone wrong here. 61.247.39.121 (talk) 00:50, 12 May 2018 (UTC)[reply]

Good point -- the numbers they tot up come out with the opposite sign. To summarize the values:

Source	C=C	O-H	C-H	C-O	total
Theirs	349	497	422	385	+39
Wikipedia	~270*	460	423	n/a	?
Statemaster	261*	460	410	350	-39

Where the sources are your image, Bond-dissociation energy, and www.statemaster.com/encyclopedia/Bond-dissociation-enthalpy (they are blacklisted, but they came up top in a search and have a handy looking table, though I have a suspicion it was once ours if I looked at the history). Note our article, or at least its BDE table as it is now, kind of sucks. Asterisks are inferred by taking C=C minus C-C.

The message I'm getting is that there's a certain amount of sage smoke and tossing of dried chicken bones involved in this computation, though I can't rule out it could give a scientific result in the hands of a competent practitioner. Wnt (talk) 00:56, 13 May 2018 (UTC)[reply]

Thanks for the response. The BDE table they give in the text book is here. I can't see where they cite their source for the data. Presumably they didn't redo weeks of calorimetry work to determine values that can easily be looked up. The text book itself is Organic Chemistry, 8th Edition by Brown, et al. 61.247.39.121 (talk) 05:31, 13 May 2018 (UTC)[reply]

They definitely used values from that table, in which the big disagreement here is C=C. They say CH2=CH2 has double bond energy of 727 kJ/mol, but Statemaster says 611 and Wikipedia has "611-632". That said, this paper works out at 728 kJ/mol (chart 1, page 6, given as 174.1 kcal/mol). (another source doesn't count because it's the book in question). A more serious source is [1] which gives the dissociation of ethylene to "^eqCH2" is 720.96 kJ/mol at 0 K and 730.59 kJ/mol at 298.15 K, with precision of +- 0.24 kJ/mol. The same for ethane is 367.87 at 0 K and 376.66 at 298.15K, with precision of +- 0.19 kJ/mol -- with a caveat that they don't specify what it dissociates to. Which gives a difference of 353.09 to 353.93 ... which vindicates your textbook where the table is concerned, if not the sign of their mathematics. Everyone must be agreeing on something else that is actually wrong! Wnt (talk) 19:49, 13 May 2018 (UTC)[reply]

When all of our errors align so as to give the false impression of a theory that accurately describes our observations of the physical nature of the universe, that's where the real science happens! 202.155.85.18 (talk) 00:26, 14 May 2018 (UTC)[reply]

In another section of the text, this passage appears. The bonds being broken in the enol are a sigma O-H and the pi bond of C=C, and to form the ketone a sigma C-H bond and the pi bond of a C=O must form. From their own table the BDE of the sigma bond being broken is 468kJ/mol and the sigma bond being formed is 439KJ/mol. That's fairly similar I guess, though 29KJ/mol is a big enough difference to drive a reaction. Their table doesn't give any values for C=O bonds, but from elsewhere (after conversion from kcal to KJ), it's apparently 351KJ/mol. Compared to their value of 349KJ/mol for the pi component of the C=C, they're far more similar in energy than the sigma bonds are. I'm starting to wonder if I should find another text book... 202.155.85.18 (talk) 00:50, 14 May 2018 (UTC)[reply]

The Bill

Yesterday I received a letter from my electricity company requesting a meter reading. It included this boxed comment:

Warning: only read your meter if it's safe to do so

Are there any hazards of reading electricity meters consumers haven't been told about (apart from possible shock experienced on opening the resultant bill)? 81.139.216.102 (talk) 09:16, 12 May 2018 (UTC)[reply]

I'm in the UK, I assume you are too. There's a certain injury rate amongst US meter readers, where they're reading US style meters that are mounted on exterior walls, in weatherproofed cases. Some of these are high up, some are obscured by overgrown shrubbery (usually kudzu). The meter reading trade press (yes, that's a thing) sometimes mentions this in their, "We can't believe what those crazy Americans get up to" columns. The UK equivalent are meters in old cellars, with failing steps down to them.

There's also a hazard for reading damaged meters, with failing insulation.

All of these are rare, but as there are so many meters, even the rare becomes common. Andy Dingley (talk) 09:24, 12 May 2018 (UTC)[reply]

Regarding Andy's last point, cf. the totalitarian principle. --76.69.47.55 (talk) 10:26, 12 May 2018 (UTC)[reply]

Condoms failing despite perfect use

Why do condoms sometimes fail to prevent pregnancy despite perfect use? — Preceding unsigned comment added by 172.116.86.16 (talk) 09:33, 12 May 2018 (UTC)[reply]

Every packet comes with an information leaflet. These leaflets explain that the product is not 100% reliable, and they also explain why. 81.139.216.102 (talk) 09:47, 12 May 2018 (UTC)[reply]

Condom#Causes_of_failure. DroneB (talk) 12:12, 12 May 2018 (UTC)[reply]

Bird

Curious about this bird, spotted in Warsaw, Poland. Had yellow or light-colored beak, greenish neck and walked faster than pigeon, not jumped. Not as timid as sparrows. Thanks. Brandmeister^talk 18:24, 12 May 2018 (UTC)[reply]

Common starling? Henry ^Flower 19:50, 12 May 2018 (UTC)[reply]

That is a set of mighty unclear photos. I can safely rule out flamingo, harpy eagle, and wandering albatross, but not much else. :-) Common starling meets your description, at least. Matt Deres (talk) 22:40, 12 May 2018 (UTC)[reply]

Most likely, thnx. Brandmeister^talk 22:49, 12 May 2018 (UTC)[reply]

Agree it's likely to be a starling - they scavenge amongst the sparrows and pigeons in London. Alansplodge (talk) 11:56, 16 May 2018 (UTC)[reply]